Coal-fired power plants typically burn pressurized coal/air streams delivered to a fireball in the combustion chamber. The coal/air stream is delivered by a powerful exhauster fan located in series between the combustion chamber and the coal mill or pulverizer, which grinds raw coal into dust-like "fines" for efficient combustion.
An example of a typical pulverizing coal mill is disclosed in U.S. Pat. No. 5,386,619 to Wark.
An example of a prior exhauster fan is disclosed in U.S. Pat. No. 5,363,776, also to Wark. This patent illustrates the pathway from the pulverizer through the fan to the combustion chamber.
Prior exhauster fans, as disclosed in the '776 patent above, typically enclose the fan blades in a housing. The housing has an inlet from the pulverizer directing coal axially into the spinning blades. The blades then redirect the coal radially in the housing, to and through an outlet to the combustion chamber. The blades themselves are heavy, usually rectangular plates of hardened steel or a combination of mild steel with a hardened liner, for example a ceramic liner. The blades are attached to a motorized hub with a strong, heavy "spider" assembly of heavy-gauge steel spokes having angle irons to which the plates are bolted with a dozen or so bolts apiece.
Referring first to prior art FIGS. 1 and 2, a prior art exhauster fan assembly 14 is shown mounted in its housing 10. Housing 10 has an inlet 12 for receiving coal fines which it draws from the pulverizer, and a radial outlet 30 through which the fan throws the coal fines to the combustion chamber. Fan 14 generally comprises a drive hub 16, typically powered via a cantilevered drive shaft 17 by a motor which is coupled simultaneously to the fan and the pulverizer drive. Fan blades 20 are attached to the hub by a spider assembly 18 having a number of integrally formed, spoke-like ribs 18a, dual angle irons 18b mounted on the end of each rib, and a number of bolts 18c used to fasten the plates directly to the angle irons 18b. The fan assembly is primarily made from thick steel, reinforced at areas of extra wear, and is extremely heavy. The fan blades 20 themselves, which may measure several feet in length, are typically manufactured from a 3/8" thick hardened steel blade, or a 1/4" to 5/16" mild steel blade with a 1/8" to 3/16" hardened ceramic liner.
To reduce turbulence and wear between the fan blades and the housing, illustrated fan 14 may be a "shrouded" fan, in which the blades are enclosed front and back with shrouds 22, 24 (phantom lines) welded or attached via angle iron and bolt structure (not shown) directly to the front and back edges of the blades to form a structurally integral unit. Shrouds 22, 24 are intended to reduce drag and turbulence between the fan blades and the adjacent walls of housing 10. Fan assembly 14 may also be provided with known "whizzer disk" and angle structure 23, 25 in addition to front shroud plate 22.
The front of hub 16 is provided with a conical or flat "Cooley" cap 28 intended to protect the hub and redirect incoming coal fines radially to the fan blades, although in practice it creates turbulence and does not effectively protect fan structure such as the ribs from erosion.
Coal mills often measure efficiency by the pounds per hour of coal fines delivered to the combustion chamber, given a fixed power input to the motor which drives both the exhauster fan and the pulverizer bowl mill. Because the output of the motor is limited, increasing efficiency requires attention to other factors, for example the ability of the fan to provide sufficient flow to keep up with the bowl mill pulverizing action and to prevent ground coal from spilling over the side of the bowl. Alternately, where the air flow provided by the existing fan design is more than sufficient, it may be desirable to reduce the horsepower supplied to the fan to increase the horsepower supplied to the bowl mill, for example where the mill's coal supply is switched from easy-to-grind soft coal to hard coal.
Related factors which affect efficiency or performance, besides the size of the fan blades, are 1) the overall weight of fan assembly 14, which requires more amperage on motor startup and draws more horsepower during operation; 2) erosion and uneven wear of the fan parts, which creates fan imbalances leading to excess vibration, bearing failure, and structural failure of the heavy fan on the end of its cantilevered drive shaft and 3) how easily the fan "breathes" in terms of smooth coal/air flow through the eye of the fan for a given horsepower.
In terms of weight, the standard spider assembly 18 with its angle irons, bolts and heavy ribs and blades is a major power draw on the motor. The angle iron and bolt attachments for the front and back shrouds are also a significant source of weight. Extra weight on the cantilevered fan shaft bearings (not shown) increases the rate of bearing failure. Also, the heavy spider assembly concentrates weight on the very end of the drive shaft and distributes it over a long moment arm radially outward from the drive shaft.
In terms of erosion, the ribs 18a of the spider assembly tend to wear significantly, especially toward the center of the fan where the Cooley cap initially diverts the abrasive coal flow into the center of the blades. The unshrouded rear inside edge 20a of the fan blades creates turbulence and drag, since air swirls turbulently in this "air gap". Ribs 18a additionally obstruct the coal flow as it enters the blade region, further reducing efficiency.
When any of the above-mentioned portions of the fan becomes significantly eroded, the fan must be taken off-line for repairs or replacements, at which point the integral structural connection of the shrouds and the fan blades, and the large number of bolts connecting each fan blade to the spider assembly, make disassembly difficult and time consuming.
Another disadvantage of the prior art spider assembly 18 is the difficulty in assembling and maintaining a symmetrical, balanced fan given the large number of angle irons and bolt-together pieces.